1. Field of the Invention
The present invention relates generally to dichromated gelatin (DCG) used in the fabrication of holographic optical elements. More particularly, the present invention relates to the use of a glass network structure within the dichromated gelatin to provide structural stability and passivation to the holographic gelatin film.
2. Description of Related Art
Dichromated gelatin has been used for at least ten years as the material of choice for recording high performance holographic optical elements or holograms. A continuing problem with dichromatic gelatin is that the material is extremely sensitive to temperature, pressure, processing parameters and water vapor. Much of the gelatin's sensitivity is due to a lack of structural integrity in the colloidal film.
In order to compensate for the gelatin's inherently weak structure and susceptibility to water vapor, etc., the gelatin films are generally isolated and supported between gas impermeable barriers. Typically, the dichromated gelatin films are laminated between microsheets of glass or plastic. In addition, chemical hardening of techniques and porous glass monoliths containing diffused monomeric recording materials have been used. However, microsheet laminates are difficult to work with, are mechanically unstable and are incompatible when curved substrate configurations are required. Further, performance degradation generally occurs due to index mismatches among the various laminate layers required to protect the dichromated gelatin film.
Chemical hardening has been somewhat successful in passivating the gelatin film against attach by moisture, etc. However, chemical hardening does not yield films which are sufficiently hardened to be of much practical use. Further, chemical hardening tends to degrade performance of highly efficient holograms. The use of porous glass structures to reinforce the dichromated gelatin is limited by the amount of photopolymerizable material which can be introduced by diffusion into the porous glass network. Generally, only 15 to 25 weight percent gelatin can be introduced by diffusion into such porous glass networks. In addition, such porous glass networks cause unacceptable light scatter.
There presently is a need to provide a means for passivating dichromated gelatin films to reduce their susceptibility to temperature, pressure, processing parameters and water vapor. In accomplishing such gelatin passivation, it is also necessary to provide a relatively lightweight, low cost structure which is sufficiently strong to be used in visor displays, head up displays, and head down displays using high gain holographic diffusing screens. Such passivation of the dichromated gelatin hologram should also be simple and efficient so that it can be used in the mass production of low cost holographic displays.